The present invention relates generally to MEMS devices, and more particularly to low temperature methods for making MEMS devices out of silicon and silicon-germanium alloys.
Advancements in micromachining and other microfabrication techniques and processes have enabled the fabrication of a wide variety of MicroElectroMechanical Systems (MEMS) and devices. These include moving rotors, gears, switches, accelerometers, miniaturized sensors, actuator systems, and other such structures.
One important application of microfabrication is in the fabrication of RF MEMS switches. Such devices have several advantages over their solid state counterparts. For example, RF MEMS switches provide lower insertion loss, higher isolation, better linearity, and lower power than solid state switches. RF MEMS devices are also useful in a variety of applications. Thus, they can be used as tunable preselectors and frequency synthesizers, and are also useful as components in a variety of telecommunications devices and systems, including signal routing devices, impedance matching networks, and adjustable gain amplifiers.
FIG. 1 and FIG. 2 (the later of which is a side view of FIG. 1) depict a conventional RF MEMS switch 10. The switch includes a cantilevered arm 20 which typically comprises an insulating material and which is attached to the substrate 12 by an anchor structure 14. The anchor structure may be formed as a mesa on the substrate by deposition buildup or through the selective removal or etching away of surrounding material. A bottom electrode 16, which is typically grounded, and a signal line 18 are also formed on the substrate. The bottom electrode and signal line typically comprise strips of a metal that is not easily oxidized, such as gold. A gap exists between the signal line and the bottom electrode.
The actuating part of the switch comprises the cantilevered arm 20 noted above. The cantilevered arm forms a suspended microbeam which is attached at one end to the top of the anchor structure and which extends over and above the bottom electrode and the signal line disposed on the substrate. An electrical contact 22, which also typically comprises a metal such as gold that does not oxidize easily, is formed on the end of the cantilever arm that is removed from the anchor structure. The electrical contact is positioned on the bottom side of the cantilever arm so as to face the top of the substrate over and above the signal line.
A top electrode 24, typically comprising a metal such as aluminum or gold, is formed atop the cantilever arm. The top electrode starts above the anchor structure and extends along the top of the cantilevered arm to end at a position above the bottom electrode. The cantilevered arm and top electrode are broadened above the bottom electrode (which is itself broadened) to form a capacitor structure 26. The capacitor structure is provided with a grid of holes to reduce its mass.
In operation, the switch is normally in an xe2x80x9cOffxe2x80x9d position as shown in FIG. 2. With the switch in the off-state, the signal line is an open circuit due to the gap between the electrical contact and the signal line. The switch is actuated to the xe2x80x9cOnxe2x80x9d position by application of a voltage on the top electrode. With a voltage on the top electrode, electrostatic forces attract the capacitor structure (and cantilever arm) toward the bottom electrode. Actuation of the cantilevered arm toward the bottom electrode, as indicated by arrow 11, causes the electrical contact to move against the signal line, thereby closing the gap and placing the signal line into the on-state (i.e., closing the circuit).
One problem encountered in devices of the type depicted in FIGS. 1 and 2 relates to the mismatch in coefficients of thermal expansion (CTEs) between the materials used for certain components of the device. In particular, in the case of RF MEMS switches, a thermal mismatch typically exists between the top electrode (which, as noted above, is typically made out of a metal such as Au) and the cantilevered arm (which is usually made out of a material such as silicon oxynitride (SiON)). As a result, the movable portion of the switch tends to become permanently distorted during the thermal cycles that occur after release and during the packaging process, thus leading to changes in the operating characteristics of the switch and, in many cases, switch failure.
A variety of other materials have been used in MEMS fabrication processes, some of which have CTEs that more closely match the CTE of SiON. However, the use of many of these materials in the top electrode of an RF MEMS switch has been precluded by the processing considerations attendant to conventional fabrication methodologies. Thus, for example, silicon and silicon/germanium alloys have been used as structural elements in MEMS processes using LPCVD, and have a number of desirable properties. However, the maximum processing temperature for a typical RF MEMS switch is limited to about 350xc2x0 C. (due primarily to the presence of the sacrificial layer, which is typically made out of a polyimide or a similar thermally sensitive material), which is well below the deposition temperatures of about 550xc2x0 C. that are required for silicon or silicon-germanium alloys in an LPCVD or epitaxial process.
Processing temperature considerations have likewise precluded the use of materials such as silicon and silicon/germanium alloys in other MEMS applications, in spite of the desirable physical and electrical properties that these materials have. Such applications include, for example, the fabrication of MEMS devices integrated with CMOS (Complimentary Metal Oxide Semiconductor) structures such as sensors and actuators. CMOS structures are very effective device configurations for the implementation of digital functions, due to their low power consumption and dissipation and the minimization of their current in the off-state. With commercial CMOS-compatible micromachining, microstructures and support circuitry can coexist on the same substrate, and thus can be fabricated in an integrated process.
However, in order to ensure proper integration into a CMOS process and good portability between generations of CMOS, it is preferable to integrate MEMS fabrication into the backend of a CMOS process. This requires formation of the MEMS structures after the interconnect metal has already been deposited. However, the presence of the interconnect metal on the substrate requires that the substrate not be exposed to temperatures in excess of 450xc2x0 C.; these temperatures are again well below the deposition temperatures of about 550xc2x0 C. that are required for silicon or silicon-germanium alloys in an LPCVD or epitaxial process. Hence, the use of these materials in backend processing of a CMOS device is precluded. Although it may be possible in some process flows to circumvent this problem by integrating the MEMS fabrication into the beginning or middle of a CMOS process, this is undesirable in that it limits the portability of the process between generations of CMOS.
There is thus a need in the art for a low temperature method for making MEMS devices or components thereof out of silicon or silicon/germanium alloys. There is also a need in the art for a method of fabricating MEMS structures or components based on these materials which can be integrated into the backend of a CMOS process, and which can be used to fabricate sensors and actuators. There is further a need in the art for an RF MEMS device, and a method for making the same, in which the CTE of the top electrode and cantilevered arm are closely matched. These and other needs are met by the devices and methodologies disclosed herein.
In one aspect, a method for making a MEMS device is provided herein. In accordance with the method, a substrate is provided, and a MEMS structure or component thereof is created on the substrate through the Plasma Assisted Chemical Vapor Deposition (PACVD) of a material selected from the group consisting of silicon and silicon-germanium alloys. The low temperatures attendant to PACVD allow these materials to be used in fabrication processes where their use would have previously been precluded by processing considerations, and also allows these materials to be doped in situ. In accordance with this method, PACVD may be used to fabricate MEMS structures (or components thereof) on a variety of substrates and in a variety of applications. Thus, for example, PACVD may be used to fabricate MEMS structures on CMOS substrates (in which case the MEMS structure could be, for example, a sensor or actuator). PACVD may also be used to fabricate MEMS structures or components (including, for example, electrode and structural elements) in an RF MEMS fabrication process.
In another aspect, a method for making a MEMS structure is provided. In accordance with the method, a CMOS substrate is provided having interconnect metal deposited thereon. The interconnect metal may comprise, for example, gold or aluminum. A MEMS structure or component thereof is created on the substrate through the plasma assisted chemical vapor deposition of a material selected from the group consisting of silicon and silicon-germanium alloys. The plasma assisted chemical vapor deposition typically occurs at a temperature of less than about 450xc2x0 C., preferably less than about 400xc2x0 C., more preferably less than about 350xc2x0 C., even more preferably less than about 300xc2x0 C., and most preferably less than about 250xc2x0 C. If desired, the material may be doped as it is deposited.
In still another aspect, a method for manufacturing microelectromechanical sensors and actuators is provided herein. In accordance with the method, a CMOS substrate is provided having at least a first surface region thereon comprising a first material selected from the group consisting of silicon, glass and gallium arsenide, and at least a second surface region thereon comprising a second material selected from the group consisting of silicon oxide and polyimide. A layer of a third material is formed over the substrate which extends over at least a portion of the first and second regions, wherein the third material is selected from the group consisting of silicon and silicon-germanium alloys, and wherein the layer of the third material is formed at a temperature of less than about 450xc2x0 C., more preferably less than about 350xc2x0 C., even more preferably less than about 300xc2x0 C., and most preferably less than about 250xc2x0 C. through a plasma assisted chemical vapor deposition process. At least a portion of the second material is removed from underneath the layer of the third material so as to form a micromechanical deflection element comprising the third material.
In yet another aspect, a method is provided herein for making an RF MEMS switch. In accordance with the method, a substrate is provided having a signal line or other circuitry defined thereon for supporting an RF MEMS switch. A sacrificial layer is applied to at least a portion of the substrate. A structural element of an RF MEMS switch is then formed over the sacrificial layer, and a top electrode is formed on the structural element through the use of a plasma assisted chemical vapor deposition process. The top electrode is formed at a temperature that is typically less than about 400xc2x0 C., more preferably less than about 350xc2x0 C., even more preferably less than about 300xc2x0 C., and most preferably less than about 250xc2x0 C. The top electrode comprises a material selected from the group consisting of silicon and silicon-germanium alloys. At least a portion of the sacrificial layer is then removed from underneath the structural element so as to release the element.
In another aspect, an RF MEMS switch is provided which has an electrode comprising a material selected from the group consisting of silicon and silicon/germanium alloys. The switch preferably comprises a cantilevered arm, and the electrode is preferably disposed on the top of the cantilevered arm. A second electrode is preferably disposed below the cantilevered arm.
In still another aspect, devices are disclosed which may be made using the above noted methodologies.
These and other aspects are described in further detail below.